Contact detection for physiological sensor

ABSTRACT

Detecting user contact with one or more electrodes of a physiological signal sensor can be used to ensure physiological signals measured by the physiological signal sensor meet waveform characteristics (e.g., of a clinically accurate physiological signal). In some examples, a mobile and/or wearable device can comprise sensing circuitry, stimulation circuitry, and processing circuitry. The stimulation circuit can drive one or more stimulation signals on one or more electrodes, the resulting signal(s) can be measured (e.g., by the sensing circuitry), and the processing circuitry can determine whether a user is in contact with the electrode(s). Additionally or alternatively, in some examples, mobile and/or wearable device can comprise saturation detection circuitry, and the processing circuitry can determine whether the sensing circuitry is saturated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(e) of U.S.Provisional Patent Application No. 62/729,590, filed Sep. 11, 2018, thecontent of which is incorporated herein by reference in its entirety forall purposes.

FIELD

This relates generally to systems and methods of processingphysiological signals, and more particularly, to detecting contact withone or more electrodes of a physiological sensor.

BACKGROUND

Electrocardiogram (ECG) waveforms can be generated based on theelectrical activity of the heart during each heartbeat. The waveformscan be recorded from multiple electrical leads attached to various areasof a patient. For example, a 12-lead ECG system with a group of tenmeasurement electrodes that can be placed across the patient's chest,and a group of ten measurement electrodes that can be attached to thepatient's limbs. The measurement electrodes for ECG data acquisition caninclude a conducting or electrolytic gel (e.g., Ag/AgCl gel) to providea continuous, electrically-conductive path between the skin and theelectrodes. Such “wet” electrodes can reduce the impedance at theelectrode-skin interface, thereby facilitating the acquisition of alow-noise ECG signal. All of the measurement electrodes can be connectedto a device where signals from the measurement electrodes can betransmitted for storage, processing, and/or displaying. Devices withnumerous “wet” electrodes coupled to the user's chest and limbs areinvasive, may be difficult to operate for a layperson, and the resultECG waveform may be difficult to interpret. As a result, ECGmeasurements and analysis may limit the usage of ECG devices to amedical setting or by medical professionals.

One method of measuring an ECG signal is to use dry electrodes that makecontact with two areas of a patient, oftentimes on opposite sides of theheart (e.g., on each of the user's hands). On a mobile device (e.g., awearable device), ECG electrodes can be placed on the device such thatthe user can make contact with two electrodes. Reliable contact may berequired to generate accurate ECG waveforms.

SUMMARY

This relates to devices and methods of using a mobile or wearable deviceto detect a user contact with one or more electrode(s) for themeasurement of a physiological signal (e.g., ECG signals) for processingand/or display on the mobile or wearable device. The mobile or wearabledevice can comprise one or more measurement electrodes, one or morereference electrodes, and processing circuitry coupled to theelectrodes. In some examples, the device can include a stimulationcircuit. The stimulation circuit can drive a stimulation signal on oneof the measurement electrodes. In some examples, the processingcircuitry can detect a signal resulting from the stimulation signal and,based on the detected signal, determine whether a user is in contactwith the one or more measurement electrodes. In some examples, upondetermining that a user is in contact with the one or more measurementelectrodes, the processing circuitry can measure a physiological signalof the user.

In some examples, the stimulation circuit can drive a first stimulationsignal on one of the electrodes (e.g., a first measurement electrode)and a second stimulation signal on a second of the electrodes (e.g., afirst reference electrode). In some examples, the processing circuitrycan detect one or more signals resulting from the first and secondstimulation signals and based on the one or more detected signal,determine whether a user is in contact with the one or more electrodes.In some examples, upon determining that a user is in contact with theone or more electrodes, the processing circuitry can measure aphysiological signal of the user. In some examples, while measuring thephysiological signal of the user, the simulation circuit can drive oneor more of the electrodes to determine whether the user maintainscontact with the one or more electrodes during the measurement of thephysiological signal of the user.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate example systems including a physiological sensorand in which contact detection according to examples of the disclosuremay be implemented.

FIG. 2 illustrates a block diagram of an example computing system thatillustrates one implementation of physiological signal processingaccording to examples of the disclosure.

FIGS. 3A-3B illustrate example systems of measuring physiologicalsignals according to examples of the disclosure.

FIGS. 4A-4B illustrate example systems for measuring physiologicalsignals according to examples of the disclosure.

FIGS. 5A-5B illustrate example systems for measuring physiologicalsignals and for contact detection according to examples of thedisclosure.

FIG. 6 illustrates an example system for measuring physiological signalsaccording to examples of the disclosure.

FIG. 7 illustrates an exemplary process of physiological signaldetection including contact detection and/or saturation detectionaccording to examples of the disclosure.

FIG. 8 illustrates an exemplary process of physiological signaldetection including contact detection and/or saturation detectionaccording to examples of the disclosure.

FIG. 9 illustrates an example system for measuring physiological signalsand for contact detection on multiple electrodes according to examplesof the disclosure.

FIG. 10 illustrates exemplary signal processing for contact detectionaccording to examples of the disclosure.

FIG. 11 illustrates an exemplary process of physiological signaldetection including contact detection according to examples of thedisclosure.

FIG. 12 illustrates an example system for measuring physiologicalsignals and for contact detection on multiple electrodes according toexamples of the disclosure.

DETAILED DESCRIPTION

In the following description of examples, reference is made to theaccompanying drawings which form a part hereof, and in which it is shownby way of illustration specific examples that can be practiced. It is tobe understood that other examples can be used and structural changes canbe made without departing from the scope of the disclosed examples.

This relates to devices and methods of using a mobile or wearable deviceto detect a user contact with one or more electrode(s) for themeasurement of a physiological signal (e.g., ECG signals) for processingand/or display on the mobile or wearable device. The mobile or wearabledevice can comprise one or more measurement electrodes, one or morereference electrodes, and processing circuitry coupled to theelectrodes. In some examples, the device can include a stimulationcircuit. The stimulation circuit can drive a stimulation signal on oneof the measurement electrodes. In some examples, the processingcircuitry can detect a signal resulting from the stimulation signal and,based on the detected signal, determine whether a user is in contactwith the one or more measurement electrodes. In some examples, upondetermining that a user is in contact with the one or more measurementelectrodes, the processing circuitry can measure a physiological signalof the user.

In some examples, the stimulation circuit can drive a first stimulationsignal on one of the electrodes (e.g., a first measurement electrode)and a second stimulation signal on a second of the electrodes (e.g., afirst reference electrode). In some examples, the processing circuitrycan detect one or more signals resulting from the first and secondstimulation signals and based on the one or more detected signal,determine whether a user is in contact with the one or more electrodes.In some examples, upon determining that a user is in contact with theone or more electrodes, the processing circuitry can measure aphysiological signal of the user. In some examples, while measuring thephysiological signal of the user, the simulation circuit can drive oneor more of the electrodes to determine whether the user maintainscontact with the one or more electrodes during the measurement of thephysiological signal of the user.

FIGS. 1A-1B illustrate example systems including a physiological sensorand in which contact detection according to examples of the disclosuremay be implemented. FIG. 1A illustrates an example wearable device 150(e.g., a watch) that includes an integrated touch screen 152 andphysiological sensor(s) 160 (e.g., an ECG sensing system including oneor more measurement electrodes, one or more reference electrodes, andprocessing circuitry coupled to the electrodes). Wearable device 150 canbe attached to a user using a strap 154 or any other suitable fastener.FIG. 1B illustrates an example view of the back side of wearable device150 that includes electrodes 166A-C of physiological sensor 160.Physiological sensor 160 can include electrode 166C implemented in crown162 of wearable device 150, an electrode implemented in button 164 ofwearable device 150 (not shown), electrode 166A on the back side ofwearable device 150 and/or electrode 166B on the backside of wearabledevice 150. In some examples, the physiological sensor 160 can include ameasurement electrode (e.g., electrode 166C in crown 162), a firstreference electrode (e.g., electrode 166A on the backside of wearabledevice 150) and a second, ground reference electrode (electrode 166B onthe backside of wearable device 150). In some examples, thephysiological sensor 160 can include a measurement electrode in button164 in addition to or instead of measurement electrode 166C in crown162. In some examples, the physiological sensor 160 can include morethan one measurement electrode and more than two reference electrodes.It is understood that the above physiological sensor(s) can beimplemented in other wearable and non-wearable devices, includingdedicated devices for the acquisition and/or processing of physiologicalsignals (e.g., ECG signals). It is understood that although mobiledevice 136 and wearable device 150 include a touch screen, the displayof physiological signals described herein can be performed on atouch-sensitive or non-touch-sensitive display of the device includingphysiological sensor(s) 160, of a separate device or of a standalonedisplay. Additionally it is understood that although the disclosureherein primarily focuses on ECG signals, the disclosure can alsoapplicable to other physiological signals.

In some examples, the electrodes of physiological sensors 160 can be dryelectrodes which can be measurement electrodes configured to contact askin surface and capable of obtaining an accurate signal without the useof a conducting or electrolytic gel. In some variations, one or morereference electrodes may be located on a wrist-worn device, such as abracelet, wrist band, or watch, such that the reference electrodes cancontact the skin in the wrist region, while one or more measurementelectrodes can be configured to contact a second, different skin region(e.g., a finger of a hand opposite the wrist wearing the wrist-worndevice). In some examples, the measurement electrode(s) can be locatedon a separate component from the reference electrode(s). In someexamples, some or all of the measurement electrode(s) can be located ona wrist or finger cuff, a fingertip cover, a second wrist-worn device, aregion of the wrist-worn device that can be different from the locationof the reference electrode(s), and the like. In some examples, one ormore electrodes (e.g., reference electrode or measurement electrode) maybe integrated with an input mechanism of the device (e.g., a rotatableinput device, a depressible input device, or a depressible and rotatableinput device, for example), as shown in FIG. 1B. One or more electricalsignals at the one or more measurement (and/or reference) electrodes canbe measured and processed as described in more detail herein.

FIG. 2 illustrates a block diagram of an example computing system 200that illustrates one implementation of physiological signal processingaccording to examples of the disclosure. Computing system 200 can beincluded in, for example, wearable device 150 or any mobile ornon-mobile, wearable or non-wearable computing device for physiologicalsignal analysis and/or display. Computing system 200 can include one ormore physiological sensors 202 (e.g., ECG sensors) including one or moreelectrodes to measure electrical signals (e.g., ECG signals) from aperson contacting the ECG sensor(s) electrodes, data buffer 204 (orother volatile or non-volatile memory or storage) to store temporarily(or permanently) the physiological signals from the physiologicalsensors 202, digital signal processor (DSP) 206 to analyze and processthe physiological signals, host processor 208, program storage 210, andtouch screen 212 to perform display operations (e.g., to display realtime ECG signals). In some examples, touch screen 212 may be replaced bya non-touch sensitive display.

Host processor 208 can be connected to program storage 210 to executeinstructions stored in program storage 210 (e.g., a non-transitorycomputer-readable storage medium). Host processor 208 can, for example,provide control and data signals to generate a display image on touchscreen 212, such as a display image of a user interface (UI). Hostprocessor 208 can also receive outputs from DSP 206 (e.g., an ECGsignal) and performing actions based on the outputs (e.g., display theECG signal, play a sound, provide haptic feedback, etc.). Host processor208 can also receive outputs (touch input) from touch screen 212 (or atouch controller, not-shown). The touch input can be used by computerprograms stored in program storage 210 to perform actions that caninclude, but are not limited to, moving an object such as a cursor orpointer, scrolling or panning, adjusting control settings, opening afile or document, viewing a menu, making a selection, executinginstructions, operating a peripheral device connected to the hostdevice, answering a telephone call, placing a telephone call,terminating a telephone call, changing the volume or audio settings,storing information related to telephone communications such asaddresses, frequently dialed numbers, received calls, missed calls,logging onto a computer or a computer network, permitting authorizedindividuals access to restricted areas of the computer or computernetwork, loading a user profile associated with a user's preferredarrangement of the computer desktop, permitting access to web content,launching a particular program, encrypting or decoding a message, and/orthe like. Host processor 220 can also perform additional functions thatmay not be related to touch processing and display.

Note that one or more of the functions described herein, includingcontact detection, saturation detection and/or the processing ofphysiological signals, can be performed by firmware stored in memory(e.g., in DSP 206) and executed by one or more processors (in DSP 206),or stored in program storage 210 and executed by host processor 208. Thefirmware can also be stored and/or transported within any non-transitorycomputer-readable storage medium for use by or in connection with aninstruction execution system, apparatus, or device, such as acomputer-based system, processor-containing system, or other system thatcan fetch the instructions from the instruction execution system,apparatus, or device and execute the instructions. In the context ofthis document, a “non-transitory computer-readable storage medium” canbe any medium (excluding signals) that can contain or store the programfor use by or in connection with the instruction execution system,apparatus, or device. The computer-readable storage medium can include,but is not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus or device,a portable computer diskette (magnetic), a random access memory (RAM)(magnetic), a read-only memory (ROM) (magnetic), an erasableprogrammable read-only memory (EPROM) (magnetic), a portable opticaldisc such a CD, CD-R, CD-RW, DVD, DVD-R, or DVD-RW, or flash memory suchas compact flash cards, secured digital cards, USB memory devices,memory sticks, and the like.

The firmware can also be propagated within any transport medium for useby or in connection with an instruction execution system, apparatus, ordevice, such as a computer-based system, processor-containing system, orother system that can fetch the instructions from the instructionexecution system, apparatus, or device and execute the instructions. Inthe context of this document, a “transport medium” can be any mediumthat can communicate, propagate or transport the program for use by orin connection with the instruction execution system, apparatus, ordevice. The transport medium can include, but is not limited to, anelectronic, magnetic, optical, electromagnetic or infrared wired orwireless propagation medium.

It is to be understood that the computing system 200 is not limited tothe components and configuration of FIG. 2, but can include other oradditional components (or omit components) in multiple configurationsaccording to various examples. For example, an analog-to-digitalconverter (ADC) may be added between physiological sensor 202 and DSP206 to convert the signals to the digital domain, or touchscreen 212 canbe omitted and the ECG signal or other information from the analysis andprocessing can be relayed to another device (e.g., a tablet, laptop,smartphone, computer, server, etc.) via wired or wireless connectionthat can include a display or other feedback mechanism for outputting avisual representation of the data or other notifications or information.Additionally, the components of computing system 200 can be includedwithin a single device, or can be distributed between multiple devices.

Returning back to physiological sensor(s) 202, the mobile or wearabledevice (or other device) may comprise one or more of measurementelectrodes and one or more reference electrodes. Physiological sensors202 can be in communication with DSP 206 to acquire physiologicalsignals and transmit the signals to DSP 206. In some examples, thephysiological signals can be acquired by data buffer 204 and the DSP 206can acquire a buffered sample of the physiological waveform (e.g., 3second sample, 5 second sample, 10 second sample, 30 second sample, 60second sample). In some examples, data buffer 204 can be implemented aspart of DSP 206. It should be understood that although a DSP isdescribed, other processing circuits could be used to implement theanalysis and processing described herein including a microprocessor,central processing unit (CPU), programmable logic device (PLD), and/orthe like.

Although the examples and applications of contact detection andprocessing devices and methods are described in the context ofgenerating a complete ECG waveform, it should be understood that thesame or similar devices and methods may be used to collect and processdata from the plurality of measurement electrodes and may or may notgenerate an ECG waveform. For example, the signals from thephysiological sensors 202 may facilitate the monitoring of certaincardiac characteristics (e.g., heart rate, arrhythmias, changes due tomedications or surgery, function of pacemakers, heart size, etc.) and/orECG waveform characteristics (e.g., timing of certain waves, intervals,complexes of the ECG waveform) by the DSP and/or user without generatinga complete ECG waveform. In some examples, the controller may generate asubset of the ECG waveform (e.g., one or more of the P wave, QRScomplex, PR interval, T wave, U wave). Moreover, examples of thedisclosure include contact detection and processing devices and methodsconfigured for other types of physiological signal measurementsincluding, but not limited to, EEG and EMG measurements or opticaldetermination of heart rate.

FIGS. 3A-3B illustrate example systems of measuring physiologicalsignals (e.g., an ECG waveform) according to examples of the disclosure.In FIG. 3A, wearable device 150 can be worn on the wrist of a user. Insome examples, reference electrodes 166A and 166B on the back side ofwearable device 150 can contact the wrist of the user when worn. In someexamples, wearable device 150 can measure a physiological signal when auser contacts measurement electrode 166C on crown 162 of wearable device150 with finger 304 (e.g., of a hand opposite the wrist wearing thewrist-worn device). Physiological signal 302 can be measured in responseto the contact of finger 304 with measurement electrode 166C (and thecontact between the wrist and reference electrodes 166A and 166B). Insome examples, the measured physiological signal 302 can be a clinicallyaccurate waveform (e.g., meets the specifications for a clinicallyaccurate waveform) due to the reliable contact with measurementelectrode 166C (and reliable contact with reference electrodes 166Aand/or 166B). FIG. 3B illustrates a user contact of finger 304 with thehousing of wearable device 150 instead of crown 162. In some examples,physiological signal 312 can be acquired due to coupling between thehousing of wearable device 150 and the measurement electrode 166C. Insome examples, physiological signal 312 can have a similar morphology asphysiological signal 302, but physiological signal 312 can be attenuatedas compared to physiological signal 302 (e.g., 5%, 10%, 20% attenuation,etc.). In some examples, physiological signal 312 may be unstable,noisy, and/or the amplitude and attenuation can varynon-deterministically. In some examples, the measured physiologicalsignal 312 may not be a clinically accurate waveform (e.g., does notconform to the specifications for a clinically accurate waveform) andcan be difficult to interpret or lead to misinterpretation of thephysiological signal (e.g., as compared with physiological signal 302).Contact detection, as described herein, can be used to avoid generatingand/or presenting to a user waveforms like physiological signal 312.

FIGS. 4A-4B illustrate example systems for measuring physiologicalsignals according to examples of the disclosure. In FIG. 4A, circuit 400can include processor 430 (e.g., corresponding to DSP 206 and/or hostprocessor 208), analog front end 420, measurement electrode 402 (e.g.,corresponding to measurement electrode 166C), reference electrode 404,and ground electrode 406 (e.g., corresponding to reference electrode166A and reference electrode 166B). In some examples, circuit 400resides on a mobile device (e.g., a wearable device 150). In someexamples, analog front end 420 includes amplifier 422 andanalog-to-digital converter (ADC) 424. Amplifier 422 can be adifferential amplifier coupled to measurement electrode 402 (e.g., onthe inverting input or on the non-inverting input) and to referenceelectrode 404 (e.g., on the non-inverting input or on the invertinginput). In some examples, ground electrode 406 can be coupled to analogfront end 420 to provide a shared ground reference between circuit 400and ground electrode 406 (e.g., ground electrode 406 can provide asystem ground reference voltage). In some examples, circuit 400 caninclude networks 412, 414, and 416, along the signal paths for themeasurement electrode 402, reference electrode 404, and ground electrode406, respectively. In some examples, networks 412, 414, and 416 caninclude circuit components (e.g., resistors, capacitors, inductorsand/or diodes) and/or can include impedances inherent in circuit 400(e.g., routing impedances, parasitic impedances, etc.). In someexamples, networks 412, 414 and 416 can provide electrostatic discharge(ESD) protection for the circuit 400 and/or provide safety by limitingor preventing electrical currents being applied to the user's skinand/or preventing unexpected or unintentional external signals fromentering the device and causing damage. In some examples, amplifier 422can output an amplified differential signal and analog-to-digitalconverter 424 can convert the amplified differential signal into adigital signal. In some examples, amplifier 422 can output an amplifiedsingle-ended output. In some examples, the output of analog-to-digitalconverter 424 can be a multi-bit signal (e.g., 8 bits, 12 bits, 24 bits,etc.) coupled to processor 430. The multi-bit signal can be transmittedfrom analog front end 420 to processor 430 serially or in parallel. Insome examples, analog-to-digital converter 424 can be a differentialanalog-to-digital converter and convert a differential analog input to adigital output. In some examples, analog-to-digital converter 424 can besingle-ended and convert a single-ended analog input to a digitaloutput. In some examples, differential amplifier 422 can be implementedwith two single-ended amplifiers and ADC 424 can be implemented with twoADCs (each connected to the output of one of the single-endedamplifiers).

In some examples, a user can wear the wearable device including circuit400. In such examples, reference electrode 404 and ground electrode 406can contact with the wrist of the user. When a user touches measurementelectrode 402 (e.g., electrode 166C on crown 162 of wearable device150), measurement electrode 402 can receive a physiological signal fromthe user. In FIG. 4A, the user is represented as physiological signalsource 401. In some examples, when the user touches measurementelectrode 402, a path can be created through physiological signal source401 from measurement electrode 402 and reference electrode 404 and/orground electrode 406 (e.g., from the user's finger across the user'schest to the wrist upon which the user is wearing the wearable deviceand to reference electrode 404 and/or ground electrode 406). In someexamples, contacting measurement electrode 402 can cause circuit 400 tomeasure a physiological signal (e.g., as illustrated in and describedwith respect to FIG. 3A) from physiological signal source 401.

FIG. 4B illustrates an example circuit diagram in which a user of thedevice contacts the housing of the wearable device instead of ameasurement electrode. In FIG. 4B, circuit 450 can include the samecomponents as circuit 400, the description of which is omitted forbrevity. In some examples, when the user touches the housing of thewearable device, an alternative path can be created throughphysiological signal source 451 from reference electrode 404 (e.g.,electrode 166A connected to the user's wrist) and ground electrode 406(e.g., the housing of the wearable device can be grounded to systemground via ground electrode 406). In some examples, the alternative pathcan cause physiological signal source 451 to inject a physiologicalsignal between reference electrode 404 and ground electrode 406. In someexamples, the physiological signal can cause amplifier 422 to detect andamplify a physiological signal. In such examples, processor 430 maymisinterpret the signal from the physiological sensor(s) as a properphysiological signal. However, as described above with respect to FIG.3B, the resulting physiological signal can be attenuated, unstable, orotherwise unreliable.

FIGS. 5A-5B illustrate example systems for measuring physiologicalsignals and for contact detection according to examples of thedisclosure. For ease of description, FIGS. 5A-5B focus on a measurementelectrode, a reference electrode and the analog circuitry for measuringphysiological signals and for contact detection; processing circuitry,and a ground reference electrode are not illustrated. In FIG. 5A,circuit 500 can include analog front end 520, measurement electrode 502(e.g., corresponding to measurement electrode 166C) and referenceelectrode 504 (e.g., corresponding to reference electrode 166A and/orreference electrode 166B). Analog front end 520 can include amplifier522 (e.g., similar to amplifier 422), analog-to-digital converter 528(e.g., similar to ADC 424), buffers 524 and 526, and test signalcircuitry. In some examples, buffers 524 and 526 can provide animpedance matching interface for the electrodes (e.g., matching theimpedance of the user's body contacting with the respective electrode).In some examples, buffer 524 and 526 can be designed to accommodate thelarge input impedances 512, 514 between the electrodes and the buffer524 and 526. In some examples, buffer 524 and 526 can be designed toreduce noise or interference from the input networks that may enterinputs to amplifier 522.

In some examples, the test signal circuitry (e.g., stimulation circuit)can include test signal generator 530 and capacitor 538. In someexamples, test signal generator 530 can be a square wave generator, aclock generator, a periodic signal generator or other suitable signalgenerator. In some examples, test signal generator can include a digitalto analog converter (DAC) to convert a digital signal into an analogstimulation signal. Test signal 531 (e.g., stimulation signal) generatedby test signal generator 530 can be a square wave, a sine wave, atrapezoidal wave, a saw-tooth wave or any other suitable periodicallyoscillating, non-oscillating or non-periodic (e.g., pseudo-noise signal)waveform. The test signal, regardless of waveform, can be known orpredetermined to the system to enable detection of the resultingmeasured test signal, in some examples as described herein. Test signal531 can be capacitively coupled via capacitor 538 to measurementelectrode 502. In some examples, test signal generator 530 can becontrolled by a processor (e.g., DSP 206, host processor 208, processor430). In some examples, the processor can change the frequency and/oramplitude of test signal 531 and/or enable and disable test signalgenerator 530. In some examples, the test signal generator 530 can be aclock output of processor 430.

In some examples, analog front end 520 can include an impedance network536. In some examples, impedance network 536 can be one or more discretecapacitors and/or one or more discrete resistors. In some examples,impedance network 536 can represent parasitic impedances in the system.In some examples, impedance network 536 can be one or more capacitors(including respective parasitic impedances). In some examples, capacitor538 and impedance network 536 form a voltage divider through path 534 toground and test signal 531 generated by test signal generator 530 can bedivided by the voltage divider. Buffer 524 can measure a node betweencapacitor 538 and impedance network 536. The resulting measured testsignal can be used to detect contact on measurement electrode 502.

In some examples, the amplitude (e.g., voltage level) of the measuredtest signal can depend on the load experienced by the test signal. Forexample, when a user touches measurement electrode 502, the resultingmeasured test signal can be attenuated. As illustrated in FIG. 5A,contact between a user (e.g., a finger) and measurement electrode 502can form a path 532 for test signal 531. In some examples, path 532 canbe formed through physiological signal source 501 (e.g., the body of theuser) to system ground via the ground electrode (e.g., ground electrode406 contacting the user's wrist). In some examples, a user can becontacting measurement electrode 502 with a first finger (e.g., an indexfinger) and the housing of the device with a second finger (e.g., athumb). In such cases, path 532 for test signal 531 can be formedthrough physiological signal source 501 (e.g., the body of the user) tosystem ground through the finger touching the housing of the device(e.g., the housing of the device can be grounded to system ground).Thus, path 532 can form an impedance in parallel to path 534 (throughimpedance network 536) and change the loading experienced by test signal531. In such examples, the resulting measured test signal 540 at buffer524 can be attenuated. In contrast, when a user is not touchingmeasurement electrode 502 (or is contacting the housing), the resultingmeasured test signal may not be attenuated (or may be attenuated less).As illustrated in FIG. 5B, without contact on measurement electrode 502,path 532 may not be formed to system ground. Without path 532 to systemground for test signal 540, the resulting measured test signal 542 maynot be attenuated (or may be attenuated less) than expected from thevoltage divider of capacitor 538 and impedance network 536. Comparingthe amplitude of resulting measured test signals 540 and 542, measuredtest signal 540 corresponding to contact on measurement electrode 502can be more attenuated than measured test signal 542. In some examples,test signal 531 can travel through path 532, through physiologicalsignal source 501, and into reference electrode 504 and can be detectedby buffer 526. In some examples, detection of the resulting test signalby buffer 526 can be sufficient to determine that a user is contactingwith measurement electrode 502. In some examples, a differentialmeasurement can be performed on the resulting signal detected by buffer526 and the resulting signal detected by buffer 524 to determine theamplitude level of the resulting test signal. In some examples, asingle-ended measurement can be performed to determine the amplitude ofthe resulting test signal (e.g., without using reference electrode 504and buffer 526).

In some examples, the response of test signal 531 to the load can dependon the frequency of test signal 531 and the respective impedance of thesignal paths. In some examples, the frequency of test signal 531 can bevaried to determine the load of the signal paths at the respectivefrequency (e.g., the quality of the skin-to-electrode connection as afunction of the test signal frequency can be determined). In someexamples, an initialization process can be used to select a frequencyfor differentiating between when measurement electrode 502 is contactedand when it is not contacted (e.g., a frequency for test signal 531 thatresults in an observable change in resulting test signal amplitude). Insome examples, test signal 531 can include a plurality of frequenciesconcurrently (e.g., test signal 531 can include multiple frequencycomponents). In such an example, the reactance of the system todifferent frequencies can be determined at one time.

A threshold amplitude (e.g., voltage level) can be used to determinewhether measurement electrode 502 is contacted. When the measured testsignal is less than a threshold amplitude, the system (e.g., DSP 206,host processor 208, processor 430) can determine that the measurementelectrode is contacted (e.g., sufficient skin-to-electrode couplingexists for high-quality physiological measurements). When the measuredtest signal is greater than or equal to the threshold amplitude, thesystem can determine that the measurement electrode is not contacted (orthat the housing is contacted, or that there is insufficientskin-to-electrode coupling for a high-quality physiologicalmeasurement). The threshold amplitude can be set, for example, based onempirical study of expected range of load impedance fromskin-to-electrode coupling. Additionally, the threshold amplitude can beset based on other factors including accuracy of the resulting waveformand the desired sensitivity (e.g., with respect false positives). Asdescribed herein, detecting contact with the measurement electrode canbe used to differentiate between a reliable measured physiologicalsignal (e.g., such as physiological signal 302) from an unreliablemeasured physiological signal. In some examples, the system can providea notification for the user to contact the measurement electrode tobegin measuring a physiological signal. In some examples, as describedherein, contact detection can be used as a trigger to beginphysiological signal measurements and/or as a trigger to endphysiological signal measurements. In some examples, contact detectioncan be used to assign a confidence to physiological signal during ameasurement session. In some examples, beginning physiological signalmeasurements can include acquiring the physiological signal (e.g., bydata buffer 204 and/or DSP 206), storing the physiological signal (e.g.,in program storage 210) and/or displaying the physiological signal onthe display. In some examples, when the system determines that themeasurement electrode is not contacted, the system can forego measuringthe physiological signal (e.g., powering down the circuit, discardingthe physiological signal measurements, or otherwise not process incomingsignals). In some examples, when the system determines that themeasurement electrode is not contacted, the system can still measure thephysiological signal, but with a low confidence value indicative thatthe physiological signal is low-quality (e.g., may not be reliable forone or more intended uses). In some examples, the low confidence can berepresented in a binary manner (e.g., a low-confidence/low-quality flagcan be set. In some examples, the confidence can be represented inanother manner (e.g., a probability) representative of the quality. Insome examples, when the confidence is below a threshold or when thelow-confidence/low-quality flag is set, a notification can be presentedto the user to indicate that the measured physiological signalmeasurement may be unreliable or low quality (e.g., display thephysiological signal with a visual indicator, display a notification onthe display of the device and/or any other visual feedback, and/or anaudio feedback and/or a haptic feedback and/or any other suitablefeedback mechanism).

In some examples, when a user contacts measurement electrode 502, aphysiological signal from physiological signal source 501 can entercircuit 500. In some examples, the physiological signal can be mixed orotherwise added to test signal 531 generated by test signal generator530. In some examples, the frequency of test signal 531 can be higherthan the frequency of physiological signal. For example, the frequencyspectrum of a physiological signal can be between 0.5 Hz to 40 Hz andthe frequency of test signal 531 can be 100 Hz, 135 Hz, 200 Hz, 250 Hz,400 Hz, 500 Hz, 600 Hz, or any other suitable frequency above 40 Hz. Insome examples, the frequency spectrum of a physiological signal can bebetween 0 Hz to 150 Hz and the frequency of test signal 531 can be 500Hz, 600 Hz, or any other suitable frequency above 150 Hz. In someexamples, the amplitude of test signal 531 can be smaller than theamplitude of the physiological signal. In such examples, thephysiological signal can act as a carrier wave for test signal 531(e.g., in a manner similar to amplitude modulation). In someembodiments, a filter (e.g., a high-pass filter or a band-pass filter)can be used to filter the physiological signal and leave the test signal(e.g., test signal 540) to be compared against the threshold todetermine whether measurement electrode 502 is contacted.

In some examples, after contact with measurement electrode 502 isdetermined, test signal generator 530 can stop generating test signal531. In such examples, stopping test signal generation can save powerand/or reduce or eliminate the need for filtering (of the test signalfrom the physiological signal). In some examples, even after contactwith measurement electrode 502 is determined, test signal generator 530continues providing test signal 531. In such examples, a filter (e.g., alow-pass filter or band-pass filter) can be used to filter test signal531 and leave the physiological signal for measurement and/orprocessing. In some examples, continuing to generate test signal 531 canallow the system (e.g., DSP 206, host processor 208, processor 430) tocontinue to determine that the user is contacting measurement electrode502. In some examples, when the user of the device stops contact withmeasurement electrode 502, the system can determine that contact hasstopped and cease measuring and/or processing the physiological signal.In some examples, the system can provide a notification to the userregarding the termination of contact with the measurement electrodeduring a physiological signal measurement session. In some examples,test signal generation can be periodically restarted to determinewhether measurement electrode 502 is contacted. In some examples,contact detection can be continuous (e.g., the test signal can begenerated at all times), periodic (e.g., generated once a second, once aminute, once an hour), or may be generated in response to a trigger(e.g., launching a physiological signal application, beginning aphysiological signal measurement session, while a wearable device isdetermined to be worn, etc.).

Although FIGS. 5A-5B illustrate the integration of the test signalcircuitry with the physiological signal measurement circuitry, it isunderstood that test signal circuitry can be implemented in a differentmanner. For example, the test signal circuitry can include an amplifieror other front end circuitry (e.g., separate from amplifier 524,amplifier 522, etc.) to perform the functions of measuring the testsignal and performing contact detection. In some examples (e.g., asillustrated in FIG. 12), separate signal paths can be used for contactdetection and physiological sensing (e.g., not integrating the testsignal circuitry with the physiological signal measurement circuitry).In some examples, implementing contact detection and physiologicalsensing separately can allow for optimization of the circuitry forcontact detection for the frequencies, signal range and/or signalprecision of the test signal for contact detection and optimization ofthe circuitry for the frequencies, signal range and/or signal precisionfor physiological sensing. In some examples a switching circuit can beprovided to couple the test signal circuitry (e.g., test signalgenerator and measurement amplifier) to the measurement electrode duringcontact detection and to decouple the measurement electrode from thetest signal circuitry during the physiological signal measurement. Insome examples, the test signal circuitry can be integrated withsaturation detection circuitry, as will be described below.Additionally, although illustrated as a discrete source in FIG. 5A-5B,test signal 531 can be generated by a processor (e.g., DSP 206, hostprocessor 208). In some examples, the same processor can also be coupledto receive the measured test signals from the output of buffer 524 oranother buffer or amplifier circuit.

Furthermore, although FIGS. 5A-5B illustrate the integration of the testsignal circuitry onto the signal path of measurement electrode 502, itis understood that similar test signal circuitry can be integrated ontothe signal path of reference electrode 504 to detect contact between theuser (e.g., wrist) and reference electrode 504 in a similar manner(e.g., as illustrated in FIG. 12). Additionally, although FIGS. 5A-5Billustrate one measurement electrode 502 and one reference electrode504, in some examples, the system can have a plurality of measurementelectrodes and/or a plurality of reference electrodes, and similar testsignal circuitry can be integrated with some or all of these electrodes.

FIG. 6 illustrates an example system for measuring physiological signals(and for contact detection and/or saturation detection) according toexamples of the disclosure. In some examples, circuit 600 can be similarto circuit 500 (including impedance networks 612 and 614 correspondingto impedance networks 512 and 514, amplifier 622 corresponding toamplifier 522, ADC 628 corresponding to ADC 528, buffers 624 and 626corresponding to buffers 524 and 526, and test signal circuitryincluding test signal generator 631 and capacitor 637 corresponding totest signal generator 530 and capacitor 538, and impedance network 635corresponding to impedance network 536), but analog front end 620includes saturation detection circuit 630. In some examples, saturationdetection circuit 630 includes buffers 634 and 636, multiplexer 632, andanalog-to-digital converter 638. In some examples, buffers 634 and 636are coupled to route signals from measurement electrode 602 andreference electrode 604, respectively, to multiplexer 632. In someexamples, multiplexer 632 multiplexes between selecting the signal frommeasurement electrode 602 to pass through to processor 650 and selectingthe signal from reference electrode 604 to pass through to processor650. In some examples, processor 650 can control the multiplexing ofmultiplexer 632. In some examples, analog-to-digital converter 638converts the analog signal from multiplexer 632 to a digital signal. Insome examples, the digital signal is then input to processor 650. Insome examples, the digital output of analog-to-digital converter 638 canbe a multi-bit signal (e.g., 4 bits, 6 bits, 8 bits, 10 bits, 12 bits,etc.). In some examples, the digital output of analog-to-digitalconverter 638 can have fewer bits than analog-to-digital converter 628,as the precision of the measurement for saturation may be less forsaturation detection than for measuring the physiological signal. Insome examples, rather than time-multiplexing the measurement of signalsfor saturation detection, multiplexer 632 can be omitted and each ofbuffers 634 and 636 can be coupled to its own ADC (not shown). In someexamples, saturation detection circuit 630 can measure the signal frommeasurement electrode 602 and reference electrode 604 to determine(e.g., at processor 650) whether the corresponding measurement circuitryhas saturated. For example, the incoming signal (e.g., a physiologicalsignal from a user, or other non-physiological signals) can have anamplitude beyond the supported dynamic range of the electrode orbuffers. In some examples, if the incoming signal has saturated themeasurement circuitry (e.g., buffers 624 and/or 626), the resultingsignal can be distorted (e.g., clipped) or otherwise transformed, andlikely unusable for reliable measurement. When one or both inputs aresaturated, the device can forego measuring the physiological signal(e.g., power down some or all of the circuit, such as amplifier 622, ADC628, etc.) or otherwise not process or store the incoming signal.

As described above, in some examples, saturation detection circuit 630can be used for contact detection (e.g., as described with reference toFIG. 12 using similar circuitry as shown in saturation detection circuit630 for contact detection). For example, while the test signal isapplied by test signal circuitry, the resulting signal can be measuredby buffer 634 in saturation detection circuit 630, can be converted intoa digital signal by ADC 638 and transmitted to processor 650 for contactdetermination based on attenuation of the measured test signal.

FIG. 7 illustrates an exemplary process 700 of physiological signaldetection including contact detection and/or saturation detectionaccording to examples of the disclosure. Process 700 can be performed byone or more processors of the system (e.g., DSP 206, host processor 208,processor 650, etc.) programmed to perform process 700. At 702, astimulation signal can be driven on a measurement electrode. In someexamples, the stimulation signal (e.g., test signal 531) can be drivenby a stimulation circuit (e.g., test signal circuit) that is coupled toa measurement electrode (e.g., similar to the test signal circuitrydescribed with respect to FIGS. 5A-5B). At 704, the system can sense oneor more signals measured by a first sensing circuit and a second sensingcircuit (e.g., corresponding to amplifier/buffer 524 and 526). In someexamples, the first sensing circuit can receive the one or more signalsfrom a measurement electrode and/or a stimulation circuit and caninclude a buffer (e.g., corresponding to amplifier/buffer 524). In someexamples, the one or more signals measured by the first sensing circuitinclude a physiological signal injected via a user contact with themeasurement electrode. In some examples, the one or more signalsmeasured by the first sense circuit can include a signal measured inresponse to the stimulation signal (e.g., the resulting test signals 540or 542). In some examples, the second sensing circuit can receive theone or more signals from a reference electrode and can include a buffer(e.g., corresponding to amplifier/buffer 526). In some examples, the oneor more signals measured by a second sense circuit can represent areference voltage level of a user's body. In some examples, depending onthe physiology and the impedance of the user, the one or more signalsmeasured by a second sense circuit can include a physiological signalinjected via a user contact with the measurement electrode (e.g., thusclosing a circuit loop between the measurement electrode and thereference electrode, as described with respect to FIGS. 4A-5B).

At 706, in accordance with the one or more signals measured by the firstand second sensing circuit meeting one or more criteria (e.g., asdetermined by processor 650), the system can measure a physiologicalsignal at 714, or in accordance with the one or more signals measured bythe first and second sensing circuit not meeting one of more criteria(e.g., as determined by processor 650), the system can forgo measuring aphysiological signal at 712. As described above with respect to FIGS.5A-5B, measuring the physiological signal can include acquiring thephysiological signal (e.g., by data buffer 204 and/or DSP 206), storingthe physiological signal (e.g., in program storage 210) and/ordisplaying the physiological signal on the display (e.g., touch screen212). In some examples, forgoing measuring a physiological signal caninclude powering down the circuitry, discarding any stored signalmeasurements, or otherwise not processing incoming signals. In someexamples, the system can provide a notification for the user of thedevice to contact with the measurement electrode to begin measurement ofthe physiological signal. In some examples, the notification can be anotification displayed on the display of the device and/or any othervisual feedback, and/or an audio feedback and/or a haptic feedbackand/or any other suitable feedback mechanism. In some examples, thesystem can wait for a threshold amount of time for the signals tomeeting the one or more criteria (e.g., wait for the user to contact themeasurement electrode and/or wait for the signals to no longer besaturated). In some examples, after a timeout threshold, the system canforgo measuring the physiological signal.

In some examples, the one or more criteria optionally includes (708) acriterion that requires (e.g., that is satisfied when) a signal of theone or more signals detected in response to the stimulation signal(e.g., the measured test signal) is less than a threshold. For example,contact with the measurement electrode can be indicated by the resultingmeasured test signal measured in response to driving the stimulationsignal being below a threshold value (corresponding to resultingmeasured test signal 540). While the user is contacting the measurementelectrode, the system can measure the physiological signal that isintroduced into the system (as another of the one of the one or moresignals) via the user's contact with the measurement electrode. When theresulting measured test signal is not below a threshold value(corresponding to resulting measured test signal 542), the system canforgo measuring a physiological signal (as described above with respectto FIGS. 5A-5B).

In some examples, the one or more criteria optionally includes (710) acriterion that requires (e.g., that is satisfied when) the output of thefirst sensing circuit and the output of the second sensing circuit arenot saturated. In some examples, a saturation detection circuit caninclude circuitry (e.g., saturation detection circuit 630) coupled tothe output of the first sensing circuit and the output of the secondsensing circuit. In some examples, the saturation detection circuit caninclude buffers (e.g., buffers 634, 636), a multiplexer (e.g.,multiplexer 632) and an analog-to-digital converter (e.g., ADC 638) toconvert the signals from the buffers to a digital signal. In someexamples, the saturation detection circuit and processor 650 candetermine whether the outputs of the first sensing circuit and/or thesecond sensing circuit are saturated. For example, when the measuredvoltage is at the power supply voltage of the first/second sensingcircuit for a threshold period of time, the first/second sensing circuitcan be determined by processor 650 to be saturated. Otherwise thefirst/second sensing circuit can be determined to be non-saturated. Insome examples, whether the first or second sensing circuit is saturationcan be determined based on other characteristics (e.g., the morphologyof the measured signal). In some examples, when the outputs of the firstand second sensing circuit are both not saturated, then the system canmeasure the physiological signal. In some examples, when the outputs ofone or both of the first sensing circuit and second sensing circuit aresaturated, then the system can forgo measuring the physiological signal.In some examples, the one or more criteria can include both criteria 708and 710, can include only one criterion, or can include criteria otherthan criteria 708 and 710. In some examples, the contact and/orsaturation detection can be performed continuously to indicate thequality of a physiological signal measurement during physiologicalsignal measurement. In some examples, the contact and/or saturationdetection can be used to terminate a physiological signal measurementsession. In some examples, the contact and/or saturation detection canbe performed and the results can be used to trigger a physiologicalsignal measurement session.

FIG. 8 illustrates an exemplary process 800 of physiological signaldetection including contact detection and/or saturation detectionaccording to examples of the disclosure. Process 800 can be performed byone or more processors of the system (e.g., DSP 206, host processor 208,processor 650, etc.) programmed to perform process 800. At 802, thesystem can receive a user input requesting a physiological signalmeasurement. In some examples, the user input can be a user opening anapplication for measuring or viewing a physiological signal. In someexamples, the user input can be a request to begin a physiologicalsignal measurement session. A session can be a predefined period of time(e.g., 10 seconds, 30 seconds, 1 minute, etc.), during which thephysiological signal can be measured. The session can begin with theuser input and end at the conclusion of the duration. In some examples,the measured physiological signal measured during the session can beanalyzed, categorized, stored and/or displayed. At 804, in response tothe user request, the system can drive a measurement electrode (e.g.,corresponding to measurement electrode 166C, measurement electrode 402,measurement electrode 502) with a stimulation signal (similar to thediscussion of 702 above). In some examples, in response to the userrequest, the system can power up the physiological measurementcircuitry. At 806, the system can measure a signal generated in responseto the stimulation signal. In some examples, the signal can be aresulting test signal (e.g., resulting stimulation signal) that can bemeasured by sense circuitry. In some examples, the sense circuitry canbe a buffer coupled to a stimulation circuit and the measurementelectrode (e.g., buffer 524). In some examples, the resulting testsignal can be measured from the output of a differential amplifier(e.g., differential amplifier 522). In some examples, the signalgenerated in response to the stimulation signal (e.g., resulting testsignal) can be a divided (e.g., by a voltage divider described withrespect to FIGS. 5A-5B) version of the stimulation signal and/or can befiltered (e.g., high pass or band-pass filtered) to excludephysiological signal measurements on the measurement electrode. At 808,in response to measuring that the signal (e.g., resulting test signal)is less than a threshold voltage (e.g., as determined by a processor,such as processor 650), the system can begin measurement of aphysiological signal. In some examples, the signal can be less than athreshold voltage when a user is contacting the measurement electrode.In some examples, beginning measuring the physiological signal caninclude acquiring the physiological signal (e.g., by data buffer 204and/or DSP 206), storing the physiological signal (e.g., in programstorage 210) and/or displaying the physiological signal on the display.In some examples, the physiological signal can be acquired from themeasurement electrode via the sense circuitry (e.g., analog front end420, 520). In some examples, the physiological signal measurement can bea differential measurement between a measurement electrode and areference electrode. For example, a differential amplifier (e.g., 422,522) can output a differential signal based on the physiological signalreceived on the measurement electrode and/or the reference electrode. Insome examples, as described with respect to FIGS. 6-7, measurement of aphysiological signal can begin after a determination that the output ofthe first and second sensing circuit are not saturated.

At 810, the stimulation signal can optionally cease being driven on themeasurement electrode. In some examples, driving the stimulation signalcan be ceased in response to measuring the signal less than a thresholdvoltage. In some examples, step 810 is optional and the stimulationsignal can be continued to be driven on the measurement electrode. At812, in response to measuring the signal greater than a thresholdvoltage, the system can optionally stop measurement of a physiologicalsignal. In some examples, while the stimulation signal is driven on themeasurement electrode and after measurement of the physiological signalhas begun, the system can determine that the signal generated inresponse to the stimulation signal is no longer less than a thresholdvoltage (e.g., is greater or equal to the threshold voltage). In someexamples, when the system determines that the signal is no longer lessthan a threshold voltage, the system can cease measurement of thephysiological signal. In some examples, this can include pausing themeasurement and providing a notification (e.g., visual and/or audioand/or haptic feedback) for the user to resume contact with themeasurement electrode. In some examples, after a threshold amount oftime, pausing the measurement can time out and measurement can beaborted. In some examples, the physiological signal measured so far canbe discarded. In some examples, ceasing measurement of the physiologicalsignal can result from a determination that the measured signal nolonger conforms to the characteristics of a physiological signal or canresult from a determination that the measured signal is inconsistentwith previously measured physiological signals (e.g., the signal hasended or the signal is subject to attenuation).

The description above primarily focuses on contact detection for oneelectrode (e.g., measurement electrode 502/602). In some examples,contact detection can be performed for multiple electrodes by driving afirst stimulation signal on one of the electrodes (e.g., a firstmeasurement electrode) and a second stimulation signal on a second ofthe electrodes (e.g., a second measurement electrode or a firstreference electrode). Contact detection on multiple electrodes can beused to improve performance of physiological signal detection forsystems including proper contact on two electrodes in a similar manneras described above for contact detection on one measurement electrode502/602. FIG. 9 illustrates an example system for measuringphysiological signals and for contact detection on multiple electrodesaccording to examples of the disclosure. Circuit 900 can be similar tocircuits 500 and 600. Circuit 900 can include a first electrode (e.g.,measurement electrode 902 corresponding to measurement electrode502/602), a second electrode (e.g. reference electrode 904 correspondingto reference electrode 504/604), impedance networks 912 and 914 (e.g.,corresponding to impedance networks 512/612 and 514/614), an analogfront end circuit 920 (e.g., corresponding to analog front end 520 or620) and a processor 950 (e.g., corresponding to processor 650). Analogfront end circuit 920 can include buffers 924 and 926 (e.g.,corresponding to buffers 524/624 and 526/626), differential amplifier922 (e.g., corresponding to amplifier 522/622), and ADC 928 (e.g.,corresponding to ADC 528/628). For ease of description, the saturationdetection circuit 630 is omitted, but it should be understood thatsaturation detection can also be included as described herein forsaturation detection.

Circuit 900 can also include test signal circuitry. However, unlike theillustration of circuits 500 and 600, the test signal circuitry caninclude circuitry to drive a first stimulation signal on a firstelectrode and a second stimulation signal (different from the firststimulation signal) on a second electrode (different from the firstelectrode). For example, the test signal circuitry can include a testsignal generator including a digital to analog converter (DAC) 942configured to output two complementary stimulation signals, S1 and S2.For example, S1 and S2 can be sinusoidal waves of the same frequencywith 180 degree phase shift between S1 and S2. In some examples, DAC 942can receive an oscillating signal and/or digital values from a memory togenerate voltage values for the waveforms of S1 and S2. The firststimulation signal can be driven onto the first electrode via capacitor937 and the second stimulation signal can be driven onto the secondelectrode via capacitor 947.

It should be understood that although S1 and S2 are described above assinusoidal waves with a 180 degree phase shift, that in some examples,the first and/or second stimulation signals can be other waveforms(e.g., square wave, trapezoidal wave, saw-tooth wave or any othersuitable wave), and/or the first and second stimulation signals can havea different phase relationship (e.g., 90 degree phase shift or any othersuitable phase shift). Additionally, in some examples, the frequency ofS1 and S2 can be the same or can be different (e.g., 1 kHz and 10 kHz).Finally, it should be understood that although DAC 942 is shown asgenerating both stimulation signals, that other circuitry can be used togenerate the stimulation signals (two single-output DACs, or other testsignal generator such as those described above with respect to FIGS.5A-5B).

Circuit 900 can also include impedance networks 935 and 945 (e.g.,similar to impedance networks 536/635) that can form voltage dividerswith capacitors 937 and 947 for the two electrodes. The voltage ofrespective stimulation signals S1 and S2 can be divided by therespective voltage divider. In some examples, impedance networks 935 and945 can include one or more discrete capacitors and/or one or morediscrete resistors, and/or can represent parasitic impedances for eachof the electrodes (modeling the electrode interface).

Buffer 924 can measure the node between capacitor 937 and impedancenetwork 935 corresponding to the first electrode (e.g., measurementelectrode 902). Buffer 926 can measure the node between capacitor 947and impedance network 945 corresponding to the second electrode (e.g.,reference electrode 904). The output of buffers 924 and 926 can be inputto the two input terminals of differential amplifier 922. The output ofdifferential amplifier 922 can represent a combination of the voltage atthe node of the first electrode and the voltage at the node of thesecond electrode. For example, due to the complimentary nature of S1 andS2, the output of differential amplifier 922 can represent the sum ofthe voltages output by buffers 924 and 926 (subject to phase shiftsintroduced by the impedance changes due to electrical system and contactbetween the user and the electrode). For other non-complimentarystimulation signals, the differential amplifier can still combineoutputs of buffers 924 and 926. The resulting output from differentialamplifier 922 can, in some examples, have a sinusoidal waveform. Theanalog output from differential amplifier 922 can be digitized by ADC928 and the digitized values can be sent to processor 950 for contactdetection (e.g., in a similar manner as described with respect to FIGS.7 and 8). It should be understood that although circuit 900 illustratedin FIG. 9 shows differential amplifier 922, it is understood that insome examples, differential amplifier 922 can be replaced by twosingle-end amplifiers and two separate ADCs (e.g., in a similarconfiguration as shown in independent contact detection circuit 1230 inFIG. 12).

For example, in a similar manner as described above, contact between auser and the first electrode can attenuate the output of buffer 924(with respect to the output without contact) and contact between theuser and the second electrode can attenuate the output of buffer 926(with respect to the output without contact). The composite signaloutput from buffer amplifier 922 can be evaluated to determine whetherit meets one or more criteria. The one or more criteria can include acriterion that requires (e.g., that is satisfied when) the compositesignal detected in response to the first and second stimulation is lessthan a threshold. When the composite digitized output of amplifier 922is less than a threshold, processor 950 can determine proper contactbetween the user and both of the electrodes (e.g., sufficient contact togenerate a physiological signal of threshold quality). When thecomposite digitized output of amplifier 922 is greater than thethreshold, processor 950 can determine at least one improper contactbetween the user and one of the electrodes (e.g., insufficient contactto generate a physiological signal of threshold quality). As a result ofdetecting the composite digitized output is less than a threshold(corresponding to proper contact at two electrodes), the system canmeasure the physiological signal and/or continue measuring thephysiological signal. As a result of detecting the composite digitizedoutput is greater than the threshold (corresponding to improper contactat one or two electrodes), the system can forgo measuring aphysiological signal and/or stop measuring the physiological signal (ordiscard the results or present a notification to the user, etc.), in asimilar manner as described herein for contact detection for onemeasurement electrode.

As described herein, in some examples, the stimulation signals forcontact detection can be continuously applied, periodically applied ormay be applied in response to a trigger. In some examples, the contactdetection can be performed continuously to indicate the quality of aphysiological signal measurement during physiological signalmeasurement. In some examples, the contact detection can be used totrigger a physiological signal measurement session and/or terminate aphysiological signal measurement session. In some examples, the contactdetection can be used to differentiate between intended contact with ameasurement electrode (e.g., on crown 162) from unintended contact withthe measurement electrode (e.g., from a user's wrist). For example,contact between crown 162 and the user's wrist may be relativelyintermittent (e.g., less than 3-5 seconds) compared with intended inputfor a physiological signal measurement that may require a thresholdduration of contact (e.g., greater than 10 seconds). Thus, a sessionstarted due to unintended wrist contact may be terminated (and/or thesession results can be discarded rather than displaying an inaccuratephysiological signal measurement).

In order to perform contact detection continuously, in some examples,the stimulation frequency can be selected to be outside the frequencyband used for physiological signal measurement. For example, asdiscussed herein, in some examples, the frequency of the stimulationsignal(s) can be higher than the frequency of physiological signal. Forexample, the frequency spectrum of a physiological signal can be lessthan 150 Hz and the frequency of stimulation signal(s) can be 500 Hz,600 Hz, or any other suitable frequency above 150 Hz. Additionally,using a sine wave rather than a square wave for the stimulation signalcan improve separation of the frequency bands (as a square wave includesfrequency content in multiple frequency ranges).

As described above, the digitized output of differential amplifier 922can be processed for contact detection. In some examples, as describedabove, the contact detection can be based on an amplitude of thedigitized output. In some examples, the contact detection can be basedon impedance calculated from the digitized output (including magnitudeand phase). The latter can be used to detect additional informationregarding impedance. FIG. 10 illustrates exemplary signal processingblock diagram 1000 for contact detection processing according toexamples of the disclosure. In some examples, signal processing blockdiagram 1000 can be implemented in a digital signal processor or otherprocessing circuit (e.g., DSP 206, processors 650/950, etc.), including,for example, application specific integrated circuits, programmabledevices (field programmable gate array, programmable logic device, etc.)or software executed by a processor. In some examples, the digitalsignal processing for contact detection can operate on the output fromanalog front end circuit 920.

The digital signal processing can include a filter block 1002, anin-phase and quadrature (IQ) demodulation block 1004, a windowing block1006, an accumulation block 1008, and a magnitude and/or phase detectionblock 1010. Filter block 1002 can optionally include a high-pass filterto remove high frequency noise and/or a low-pass filter such as adecimation/anti-aliasing filter. In some examples, a band-pass filtercan be used to remove high frequency noise and low-frequencyphysiological signals. Although illustrated as filtering in the digitaldomain, it should be understood that in some implementations filteringcan additionally or alternatively be performed in the analog domain(e.g., by analog front end circuitry 920). IQ demodulation block 1004can include two mixers (e.g., signal multipliers) to mix the inputs toIQ demodulation block 1004 with an in-phase demodulation signal and aquadrature demodulation signal. For example, if stimulation signals S1and S2 correspond to an in-phase sinusoid and a 180 degree out-of-phasesinusoid (e.g., at the same frequency), the in-phase demodulation signalcan be the same as S1 and the quadrature demodulation signal can be a 90degree phase-shifted version of S1. In some examples, the demodulationsignals applied to the mixers can be stored in and provided from amemory (e.g., ROM 1020) to multiply by a digital sine wave, which can bestored in ROM memory (e.g., in or accessible by DSP 204). In someexamples, the in-phase demodulation signal can be a delayed version ofthe in-phase stimulation signal (e.g., including a programmable delayadded to account for differences in propagation through the system). Insome examples, the quadrature demodulation signal can also be adjustedby a phase delay (e.g., using a programmable delay). The I component andQ component output by IQ demodulation block 1004 can be windowed by awindowing function at windowing block 1006. The windowing functionapplied at windowing block 1006 can include any suitable windowincluding rectangular, Taylor, triangular, Hamming, Hanning, Gaussian,Kaiser, etc. The windowed I and Q components can be accumulated byaccumulator block 1008. The windowed and accumulated I and Q componentscan be used to calculate the magnitude and/or phase at magnitude and/orphase detection block 1010. As described herein, the magnitude outputcan be calculated as √{square root over (I²+Q²)} and the phase can becalculated as

${\tan^{- 1}\frac{I}{Q}},$where I can represent the in-phase input to magnitude and/or phasedetection block 1010 and Q can represent the quadrature input tomagnitude and/or phase detection block 1010. As described herein, themagnitude can be proportional to the amplitude of the impedance and canbe compared with a threshold to determine whether the user issufficiently in contact with the electrode(s) or not.

It should be understood that the signal processing of FIG. 10 is anexample signal processing, but that variations can be made withoutdeparting from the scope of the disclosure. For example, in someexamples, the magnitude may be used for contact detection and it may beunnecessary to calculate the phase. Additionally or alternatively, someor all of the filtering can be moved to the analog domain. Additionallyor alternatively, the IQ demodulation can be implemented in the analogdomain. Additionally or alternatively, the window function can beimplemented as part of the generation of the in-phase demodulationsignal and the quadrature demodulation signal (e.g., the demodulationsignals stored in ROM 1020 can be windowed or the window function can beapplied to the output of ROM 1020 prior to IQ demodulation mixers).Additionally or alternatively, other demodulation techniques aside fromIQ demodulation can be used.

In some examples, S1 and S2 can be stimulated at different frequenciesand the signal processing of FIG. 10 can be performed for eachstimulation signal frequency. For example, one signal corresponding tomeasurement electrode 902 stimulated with S1 at frequency f1 can beprocessed by the signal processing of FIG. 10, designed to capturefrequency content at or near f1 (e.g., to filter frequency content at f2in filter block 1002) to generate a magnitude and/or phase formeasurement electrode 902. One signal corresponding to referenceelectrode 904 stimulated with S2 at frequency f2 (different from f1) canbe similarly processed by the signal processing of FIG. 10, designed tocapture frequency content at or near f2 (e.g., to filter out frequencycontent at f1 in filter block 1002) to generate a magnitude and/or phasefor reference electrode 904. This processing can be time-multiplexed inprocessor 950 (applying different filtering at filter block 1002,different demodulation signals at IQ demodulation block 1004, etc.) orprocessor 950 can include two signal processing channels to performparallel processing (applying different filtering at filter block 1002,different demodulation signals at IQ demodulation block, etc.).

FIG. 11 illustrates an exemplary process of physiological signaldetection including contact detection according to examples of thedisclosure. Process 1100 can be performed by one or more processors ofthe system (e.g., DSP 206, host processor 208, processor 950, etc.)programmed to perform process 1100. At 1105, the system (e.g., DAC 942controlled by processor 950) can drive a first stimulation signal (e.g.,S1) on a first electrode (e.g., measurement electrode 902) and drive asecond stimulation signal (S2) on a second electrode (e.g., referenceelectrode 904). As described above, S1 and S2 can be differentstimulation signals. In some examples, S1 and S2 can be complementarystimulation signals with the same frequency, but 180 degreesout-of-phase. In some examples, the stimulation can be continuous,periodic or in response to a trigger (e.g., a user input requesting aphysiological signal measurement, opening an application for measuringor viewing a physiological signal).

At 1110, the system (e.g., sense circuitry) can measure one or moresignals generated in response to the stimulation signals. In someexamples, the sense circuitry can include a first buffer coupled to afirst electrode (e.g., buffer 924 coupled to measurement electrode 902)and a second buffer coupled to a second electrode (e.g., buffer 926coupled to reference electrode 904). In some examples, measurement canbe from the output of a differential amplifier (e.g., differentialamplifier 922). At 1115, the system (e.g., processor 950) can estimatecontact at the first electrode (e.g., measurement electrode 902) and/orat the second electrode (e.g., reference electrode 904). For example,the output of a differential amplifier 922 can be processed (e.g.,according to signal processing in FIG. 10) to determine a magnitude.When the magnitude is less than a threshold (e.g., as determined by aprocessor, such as processor 950), the system can estimate contactbetween the user and the first electrode and the second electrode. As aresult, the system can begin or continue measurement of a physiologicalsignal. In some examples, beginning/continuing measuring thephysiological signal can include acquiring the physiological signal(e.g., by data buffer 204 and/or DSP 206), storing the physiologicalsignal (e.g., in program storage 210) and/or displaying thephysiological signal on the display. In some examples, the physiologicalsignal can be acquired from the measurement electrode and referenceelectrode via the sense circuitry (e.g., analog front end 920). In someexamples, the physiological signal measurement can be a differentialmeasurement between a measurement electrode and a reference electrode.For example, a differential amplifier (e.g., 922) can output adifferential signal based on the physiological signal received on themeasurement electrode and/or the reference electrode. In some examples,as described with respect to FIGS. 6-7, measurement of a physiologicalsignal can begin after a determination that the output of the first andsecond sensing circuit are not saturated.

When the magnitude is greater than the threshold (e.g., as determined bya processor, such as processor 950), the system can estimate contact isweak or broken between the user and the first electrode and/or thesecond electrode. In some examples, in response to measuring the signalgreater than a threshold voltage, measurement of a physiological signalcan be stopped. In some examples, this can include pausing themeasurement and providing a notification (e.g., visual and/or audioand/or haptic feedback) for the user to resume contact with theelectrode(s). In some examples, after a threshold amount of time,pausing the measurement can time out and measurement can be aborted. Insome examples, the physiological signal measured so far can bediscarded.

Although process 1100 is described as estimating contact with the firstmeasured electrode and/or the second measurement electrode, it isunderstood that the system may take action (e.g.,starting/continuing/terminating/pausing/discarding a physiologicalsignal measurement) in accordance with the measured signal magnitudebeing greater than or less than a threshold, without making an estimateof contact. However, the threshold can be set such that the signal belowthe threshold indicates contact (e.g., sufficient contact for aphysiological signal measurement of a threshold quality) on multipleelectrodes and signal above the threshold indicates a break in contactor weak contact (e.g., insufficient contact for a physiological signalmeasurement of a threshold quality) on one or multiple electrodes.

In some examples, S1 and S2 can be at different frequencies. In somesuch examples, the processing at 1115 can include separately estimatingcontact at the first electrode (e.g., measurement electrode 902) andcontact at the second electrode (e.g., reference electrode 904). Forexample, the output of a differential amplifier 922 can be processed(e.g., according to signal processing in FIG. 10) for differentfrequencies to determine a magnitude for electrode (as describedherein). When the magnitude is less than a threshold for a respectiveelectrode (e.g., as determined by a processor, such as processor 950),the system can estimate contact between the user and the respectiveelectrode. When the magnitude is greater than the threshold for arespective electrode (e.g., as determined by a processor, such asprocessor 950), the system can estimate poor contact or a lack ofcontact between the user and the respective electrode. The system canbegin or continue measurement of a physiological signal when propercontact is established for both electrodes (e.g., when both are lessthan the threshold). When the magnitude is greater than the thresholdfor either respective contact, measurement of a physiological signal canbe stopped. In some examples, this can include pausing the measurementand providing a notification (e.g., visual and/or audio and/or hapticfeedback) for the user to resume contact with the electrode(s). In someexamples, the notifications provided to the user can change to providethe user better information about which contact to improve (e.g.,instruct the user to improve contact on crown 162/measurement electrode166C when there is poor user contact with the crown, tighten strap 154when there is poor user contact with a reference electrode 166A/166B, orboth when there are contact problems with both).

FIG. 12 illustrates an example system for measuring physiologicalsignals and for contact detection on multiple electrodes according toexamples of the disclosure. Circuit 1200 can be similar to circuits 900,including a first electrode (e.g., measurement electrode 1202corresponding to measurement electrode 902), a second electrode (e.g.reference electrode 1204 corresponding to reference electrode 904),impedance networks 1212 and 1214 (e.g., corresponding to impedancenetworks 912 and 914), an analog front end circuit 1220 (e.g.,corresponding to analog front end 920) and a processor 1250 (e.g.,corresponding to processor 950). Analog front end circuit 1220 caninclude buffers 1224 and 1226 (e.g., corresponding to buffers 924 and926), differential amplifier 1222 (e.g., corresponding to amplifier922), and ADC 1228 (e.g., corresponding to ADC 928). Circuit 1200 canalso include test signal circuitry (e.g., DAC 1242 corresponding to DAC942, coupling capacitors 1237 and 1247 corresponding to capacitors 937and 947, and impedance networks 1235 and 1245 corresponding to impedancenetworks 935 and 945).

Additionally, in order to separately perform contact detection (and/orimpedance measurements) for multiple contacts, analog front end circuit1220 can also include the independent contact detection circuit 1230.Saturation detection circuitry is omitted for ease of description, butit should be understood that saturation detection can also be includedas described herein for saturation detection (and in some examples, thesame circuitry can be used for independent contact detection and forsaturation detection).

Independent contact detection circuit 1230 can include, in someexamples, buffers 1234 and 1236, multiplexer 1232, and analog-to-digitalconverter 1238. In some examples, buffers 1234 and 1236 are coupled toroute signals from measurement electrode 1202 and reference electrode1204, respectively, to multiplexer 1232. In some examples, multiplexer1232 multiplexes between selecting the signal from measurement electrode1202 to pass through to processor 1250 and selecting the signal fromreference electrode 1204 to pass through to processor 1250. In someexamples, processor 1250 can control the multiplexing of multiplexer1232. In some examples, analog-to-digital converter 1238 converts theanalog signal from multiplexer 1232 to a digital signal. In someexamples, the digital signal is then input to processor 1250. In someexamples, the digital output of analog-to-digital converter 1238 can bea multi-bit signal (e.g., 4 bits, 6 bits, 8 bits, 10 bits, 12 bits,etc.). In some examples, the digital output of analog-to-digitalconverter 1238 can have fewer bits than analog-to-digital converter1228, as the precision of the measurement for contact detection may beless than for measuring the physiological signal. In some examples,rather than time-multiplexing the measurement of signals for contactdetection, multiplexer 1232 can be omitted and each of buffers 1234 and1236 can be coupled to its own ADC (not shown).

The signals from independent contact detection circuit 1230 can beprocessed by processor 1250. In some examples, one signal correspondingto measurement electrode 1202 and buffer 1234 can be processed by thesignal processing of FIG. 10 to generate a magnitude and/or phase shiftfor measurement electrode 1202. In some examples, one signalcorresponding to reference electrode 1204 and buffer 1236 can similarlybe processed by the signal processing of FIG. 10 to generate a magnitudeand/or phase shift for reference electrode 1204. This processing can betime-multiplexed in processor 1250 or processor 1250 can include twosignal processing channels to perform parallel processing. The magnitudeinformation for each electrode can be compared with a threshold (e.g.,in a similar manner as described herein for the differential output ofdifferential amplifier 922) to determine/estimate whether each electrodeis contacted. For example, contact between a user and the firstelectrode can attenuate the output of buffer 1224 (with respect to theoutput without contact) and contact between the user and the secondelectrode can attenuate the output of buffer 1226 (with respect to theoutput without contact).

The separate processing of FIG. 12 can allow for determining whether abreak in contact (or poor contact) is detected at the first electrode(e.g., measurement electrode 1202) or at the second electrode (e.g.,reference electrodes 1204) or both. In some examples, the notificationsprovided to the user can change to provide the user better informationabout which contact to improve (e.g., instruct the user to improvecontact on crown 162/measurement electrode 166C when there is poor usercontact with the crown, tighten strap 154 when there is poor usercontact with a reference electrode 166A/166B, or both when there arecontact problems with both).

As discussed above, aspects in of the present technology include thegathering and use of physiological information. The technology may beimplemented along with technologies that involve gathering personal datathat relates to the user's health and/or uniquely identifies or can beused to contact or locate a specific person. Such personal data caninclude demographic data, date of birth, location-based data, telephonenumbers, email addresses, home addresses, and data or records relatingto a user's health or level of fitness (e.g., vital signs measurements,medication information, exercise information, etc.).

The present disclosure recognizes that a user's personal data, includingphysiological information, such as data generated and used by thepresent technology, can be used to the benefit of users. For example, auser's heart rate may allow a user to track or otherwise gain insightsabout their health or fitness levels.

The present disclosure contemplates that the entities responsible forthe collection, analysis, disclosure, transfer, storage, or other use ofsuch personal data will comply with well-established privacy policiesand/or privacy practices. In particular, such entities should implementand consistently use privacy policies and practices that are generallyrecognized as meeting or exceeding industry or governmental requirementsfor maintaining personal information data private and secure. Suchpolicies should be easily accessible by users, and should be updated asthe collection and/or use of data changes. Personal information fromusers should be collected for legitimate and reasonable uses of theentity and not shared or sold outside of those legitimate uses. Further,such collection/sharing should require receipt of the informed consentof the users. Additionally, such entities should consider taking anyneeded steps for safeguarding and securing access to such personalinformation data and ensuring that others with access to the personalinformation data adhere to their privacy policies and procedures.Further, such entities can subject themselves to evaluation by thirdparties to certify their adherence to widely accepted privacy policiesand practices. The policies and practices may be adapted depending onthe geographic region and/or the particular type and nature of personaldata being collected and used.

Despite the foregoing, the present disclosure also contemplatesembodiments in which users selectively block the collection of, use of,or access to, personal data, including physiological information. Forexample, a user may be able to disable hardware and/or software elementsthat collect physiological information. Further, the present disclosurecontemplates that hardware and/or software elements can be provided toprevent or block access to personal data that has already beencollected. Specifically, users can select to remove, disable, orrestrict access to certain health-related applications collecting users'personal health or fitness data

Therefore, according to the above, some examples of the disclosure aredirected a device. The device can comprise sensing circuitry configuredto sense a physiological signal, the sensing circuitry including a firstsensing circuit configured to sense a first electrode and a secondsensing circuit configured to sense a second electrode; a stimulationcircuit configured to drive a stimulation signal on the first electrode;and processing circuitry coupled to the sensing circuitry, theprocessing circuitry configured to (e.g., programmed to): detect one ormore signals measured by the first sensing circuit, wherein at least onesignal of the one or more signals is measured by the first sensingcircuit in response to the stimulation signal; detect one or moresignals measured by the second sensing circuit; in accordance with theone or more signals measured by the first sensing circuit and the one ormore signals measured by the second sensing circuit meeting one or morecriteria, measure the physiological signal; and in accordance with theone or more signals measured by the first sensing circuit and the one ormore signals measured by the second sensing circuit failing to meet theone or more criteria, forgo measuring the physiological signal.

Additionally or alternatively, in some examples, the one or morecriteria can include a first criterion that requires the at least onesignal measured in response to the stimulation signal has an amplitudeless than a threshold voltage. Additionally or alternatively, in someexamples, the stimulation circuit can comprise: a signal generatorconfigured to generate the stimulation signal; and a capacitorconfigured to couple the stimulation signal to the first electrode.Additionally or alternatively, in some examples, the stimulation signalcan be a periodic oscillating signal. Additionally or alternatively, insome examples, the stimulation signal can have a frequency greater than40 Hz. Additionally or alternatively, in some examples, the stimulationsignal can have a frequency between 100 Hz and 600 Hz. Additionally oralternatively, in some examples, the sensing circuitry can furthercomprise a differential analog-to-digital converter (ADC) configured toconvert differential analog output of the differential amplifier into adigital output. Additionally or alternatively, in some examples, thesensing circuitry can further comprise two single-ended amplifiers andtwo single-ended analog-to-digital converters (ADC) configured toconvert analog output of the two single-ended amplifiers into digitaloutputs. Additionally or alternatively, in some examples, the sensingcircuitry can further comprise a saturation detection circuit coupled toan output of the first sensing circuit and an output of the secondsensing circuit, wherein the saturation detection circuit is configuredto detect saturation of the output of the first sensing circuit or ofthe output of the second sensing circuit.

Additionally or alternatively, in some examples, the saturationdetection circuit can comprise: a first buffer coupled to the output ofthe first sensing circuit; a second buffer coupled to the output of thesecond sensing circuit; a multiplexer coupled to the first buffer andthe second buffer, wherein an output of the first buffer and an outputof the second buffer are coupled as inputs to the multiplexer; and ananalog-to-digital converter (ADC). Additionally or alternatively, insome examples, the one or more criteria can include a second criterionthat requires the output of the first sensing circuit and the output ofthe second sensing circuit are not saturated. Additionally oralternatively, in some examples, the sensing circuitry can furtherinclude a differential amplifier, wherein an output of the first sensingcircuit is coupled to a first input (e.g., inverting input) of thedifferential amplifier and wherein an output of the second sensingcircuit is coupled to a second input (e.g., non-inverting input) of thedifferential amplifier.

Additionally or alternatively, in some examples, the processingcircuitry can be further configured to: in accordance with the at leastone signal of the one or more signals measured by the first sensingcircuit in response to the stimulation signal meeting the one or morecriteria, cease driving the stimulation signal. Additionally oralternatively, in some examples, the processing circuitry can be furtherconfigured to: determine, while measuring the physiological signal, thatat least one signal of the one or more signals measured by the firstcircuit in response to the stimulation signal fails to meet the one ormore criteria; and in response to determining that the at least onesignal of the one or more signals measured by the first circuit inresponse to the stimulation signal fails to meet the one or morecriteria, cease measuring the physiological signal. Additionally oralternatively, in some examples, the stimulation circuit can be drivingthe stimulation signal on the first electrode while measuring thephysiological signal. Additionally or alternatively, in some examples,measuring the physiological signal can comprise: filtering the one ormore signals measured by the sensing circuitry to remove the at leastone signal measured in response to the stimulation signal from the oneor more signals.

Some examples of the disclosure are directed to a method. The method cancomprise receiving a user input requesting a physiological signalmeasurement; in response to receiving the user input, driving a firstmeasurement electrode with a stimulation signal; measuring one or moresignals, wherein at least one signal of the one or more signals ismeasured in response to the stimulation signal; in accordance with theone or more signals meeting one or more criteria, the one or morecriteria including a criterion that requires the at least one signalmeasured in response to the stimulation signal has an amplitude lessthan a threshold voltage, performing the physiological signalmeasurement; and in accordance with the one or more signals failing tomeet the one or more criteria, forgoing the physiological signalmeasurement.

Additionally or alternatively, in some examples, the stimulation signalcan be a periodic oscillating signal. Additionally or alternatively, insome examples, the stimulation signal can have a frequency greater than40 Hz. Additionally or alternatively, in some examples, the stimulationsignal can have a frequency between 100 Hz and 600 Hz. Additionally oralternatively, in some examples, the method can further comprisedetecting saturation of an output of a first sensing circuit coupled tothe first measurement electrode or saturation of an output of a secondsensing circuit coupled to a reference electrode. Additionally oralternatively, in some examples, the one or more criteria includes acriterion that requires the output of the first sensing circuit and theoutput of the second sensing circuit are not saturated.

Additionally or alternatively, in some examples, the method can furthercomprise: in accordance with the one or more signals meeting one or morecriteria, ceasing driving the first measurement electrode with thestimulation signal. Additionally or alternatively, in some examples, themethod can further comprise: determining, while performing thephysiological signal measurement, that the at least one signal of theone or more signals measured in response to the stimulation signal hasan amplitude not less than the threshold voltage; and in response todetermining that the at least one signal of the one or more signalsmeasured in response to the stimulation signal has an amplitude not lessthan the threshold voltage, ceasing performing the physiological signalmeasurement. Additionally or alternatively, in some examples, the firstmeasurement electrode can be driven with the stimulation signal whileperforming the physiological signal measurement. Additionally oralternatively, in some examples, performing the physiological signalmeasurement can comprise: filtering the one or more signals measured bythe sensing circuitry to remove the at least one signal measured inresponse to the stimulation signal from the one or more signals.

Some examples of the disclosure are directed to non-transitory computerreadable storage medium. The non-transitory computer readable storagemedium can store instructions, which when executed by a devicecomprising a first measurement electrode and one or more processingcircuits, cause the one or more processing circuits to perform a method.In some examples, the method can comprise receiving a user inputrequesting a physiological signal measurement; in response to receivingthe user input, driving a first measurement electrode with a stimulationsignal; measuring one or more signals, wherein at least one signal ofthe one or more signals is measured in response to the stimulationsignal; in accordance with the one or more signals meeting one or morecriteria, the one or more criteria including a criterion that requiresthe at least one signal measured in response to the stimulation signalhas an amplitude less than a threshold voltage, performing thephysiological signal measurement; and in accordance with the one or moresignals failing to meet the one or more criteria, forgoing thephysiological signal measurement.

Additionally or alternatively, in some examples, the stimulation signalcan be a periodic oscillating signal. Additionally or alternatively, insome examples, the stimulation signal can have a frequency greater than40 Hz. Additionally or alternatively, in some examples, the stimulationsignal can have a frequency between 100 Hz and 600 Hz. Additionally oralternatively, in some examples, the method can further comprisedetecting saturation of an output of a first sensing circuit coupled tothe first measurement electrode or saturation of an output of a secondsensing circuit coupled to a reference electrode. Additionally oralternatively, in some examples, the one or more criteria includes acriterion that requires the output of the first sensing circuit and theoutput of the second sensing circuit are not saturated.

Additionally or alternatively, in some examples, the method can furthercomprise: in accordance with the one or more signals meeting one or morecriteria, ceasing driving the first measurement electrode with thestimulation signal. Additionally or alternatively, in some examples, themethod can further comprise: determining, while performing thephysiological signal measurement, that the at least one signal of theone or more signals measured in response to the stimulation signal hasan amplitude not less than the threshold voltage; and in response todetermining that the at least one signal of the one or more signalsmeasured in response to the stimulation signal has an amplitude not lessthan the threshold voltage, ceasing performing the physiological signalmeasurement. Additionally or alternatively, in some examples, the firstmeasurement electrode can be driven with the stimulation signal whileperforming the physiological signal measurement. Additionally oralternatively, in some examples, performing the physiological signalmeasurement can comprise: filtering the one or more signals measured bythe sensing circuitry to remove the at least one signal measured inresponse to the stimulation signal from the one or more signals.

Some examples of the disclosure are directed a device. The device cancomprise sensing circuitry configured to sense a physiological signal,the sensing circuitry including a first sensing circuit configured tosense a first electrode and a second sensing circuit configured to sensea second electrode; a stimulation circuit configured to drive a firststimulation signal on the first electrode and configured to drive asecond stimulation signal on the second electrode; and processingcircuitry coupled to the sensing circuitry. The processing circuitry canbe programmed to: in accordance with one or more signals measured inresponse to the first stimulation signal and the second stimulationsignal meeting one or more criteria, measure the physiological signal;and in accordance with the one or more signals measured in response tothe first stimulation signal and the second stimulation signal failingto meet the one or more criteria, forgo measuring the physiologicalsignal. Additionally or alternatively, in some examples, the sensingcircuitry can further include a differential amplifier. An output of thefirst sensing circuit can be coupled to a first input of thedifferential amplifier and an output of the second sensing circuit canbe coupled to a second input of the differential amplifier. The one ormore signals measured in response to the first stimulation signal andthe second stimulation signal can be output by an output of thedifferential amplifier. Additionally or alternatively, in some examples,the one or more criteria can include a first criterion that can besatisfied when the one or more signals measured in response to the firststimulation signal and the second stimulation signal have an amplitudeless than a threshold voltage. Additionally or alternatively, in someexamples, the one or more signals measured in response to the firststimulation signal and the second stimulation signal can comprise one ormore first signals measured by the first sensing circuit and one or moresecond signals measured by the second sensing circuit. Additionally oralternatively, in some examples, the one or more criteria can include afirst criterion that can be satisfied when the one or more first signalsmeasured in response to the first stimulation signal have an amplitudeless than a threshold voltage and a second criterion that can besatisfied when the one or more second signals measured in response tosecond stimulation signal have an amplitude less than the thresholdvoltage. Additionally or alternatively, in some examples, thestimulation circuit can comprise: a signal generator configured togenerate the first stimulation signal and the second stimulation signal;a first capacitor configured to couple the first stimulation signal tothe first electrode; and a second capacitor configured to couple thesecond stimulation signal to the second electrode. Additionally oralternatively, in some examples, the signal generator can comprise adigital to analog converter. Additionally or alternatively, in someexamples, the first stimulation signal can be a periodic oscillatingsignal with a first frequency and a first phase and the secondstimulation signal can be a periodic oscillating signal with the firstfrequency and a second phase, different than the first phase.Additionally or alternatively, in some examples, the first phase and thesecond phase can be separated by 180 degrees. Additionally oralternatively, in some examples, the first frequency can be greater than150 Hz. Additionally or alternatively, in some examples, the firststimulation signal and the second stimulation signal can be drivenconcurrently. Additionally or alternatively, in some examples, the firststimulation signal and the second stimulation signal can be driven atleast partially concurrently with measuring the physiological signal.Additionally or alternatively, in some examples, the first stimulationsignal can be a periodic oscillating signal with a first frequency andthe second stimulation signal can be a periodic oscillating signal withthe second frequency, different than the first frequency. Additionallyor alternatively, in some examples, the first frequency and the secondfrequency can be greater than 150 Hz. Additionally or alternatively, insome examples, the processing circuitry can be further programmed to:determine, while measuring the physiological signal, that the at leastone signal of the one or more signals measured in response to the firststimulation signal and the second stimulation signal fails to meet theone or more criteria; and in response to determining that the at leastone signal of the one or more signals measured in response to the firststimulation signal and the second stimulation fails to meet the one ormore criteria, cease measuring the physiological signal. Additionally oralternatively, in some examples, measuring the physiological signal cancomprise: filtering one or more signals measured by the sensingcircuitry to remove the one or more signals measured in response to thefirst stimulation signal and the second stimulation signal from the oneor more signals. Additionally or alternatively, in some examples, theprocessing circuitry can be further programmed to: filter the one ormore signals measured in response to the first stimulation signal andthe second stimulation signal; demodulate the one or more signalsmeasured in response to the first stimulation signal and the secondstimulation signal; window the one or more signals measured in responseto the first stimulation signal and the second stimulation signal;and/or compute an amplitude of the one or more signals measured inresponse to the first stimulation signal and the second stimulationsignal. Additionally or alternatively, in some examples, the processingcircuitry can be further programmed to: demodulate the one or moresignals measured in response to the first stimulation signal and thesecond stimulation signal with a first demodulation signal and a seconddemodulation signal, the second demodulation signal 90 degrees out ofphase with the first demodulation signal. A frequency of the firststimulation signal and the second stimulation signal can be the same asa frequency of first demodulation signal and the second demodulationsignal.

Some examples of the disclosure are directed to a method. The method cancomprise: driving a first stimulation signal on a first electrode and asecond stimulation signal on a second electrode, different from thefirst electrode; measuring one or more signals in response to the firststimulation signal and the second stimulation signal; in accordance withthe one or more signals measured in response to the first stimulationsignal and the second stimulation signal meeting one or more criteria,measuring a physiological signal; and in accordance with the one or moresignals measured in response to the first stimulation signal and thesecond stimulation signal failing to meet the one or more criteria,forgoing measuring the physiological signal. Additionally oralternatively, in some examples, measuring the one or more signals inresponse to the first stimulation signal and the second stimulationsignal can comprise: measuring the first electrode with a first sensingcircuit; and measuring the second electrode with a second sensingcircuit. The one or more signals measured in response to the firststimulation signal and the second stimulation signal can be outputs of adifferential amplifier that receives outputs of the first sensingcircuit and second sensing circuit. Additionally or alternatively, insome examples, the one or more criteria can include a first criterionthat can be satisfied when the one or more signals measured in responseto the first stimulation signal and the second stimulation signal havean amplitude less than a threshold voltage. Additionally oralternatively, in some examples, the one or more signals measured inresponse to the first stimulation signal and the second stimulationsignal can comprise one or more first signals measured by a firstsensing circuit coupled to the first electrode and one or more secondsignals measured by a second sensing circuit coupled to the secondelectrode. Additionally or alternatively, in some examples, the one ormore criteria can include a first criterion that can be satisfied whenthe one or more first signals measured in response to the firststimulation signal have an amplitude less than a threshold voltage and asecond criterion that can be satisfied when the one or more secondsignals measured in response to second stimulation signal have anamplitude less than the threshold voltage. Additionally oralternatively, in some examples, driving the first stimulation signal onthe first electrode and the second stimulation signal on the secondelectrode can comprise coupling the first stimulation signal to thefirst electrode via a first capacitor and coupling the secondstimulation signal to the second electrode via a second capacitor.Additionally or alternatively, in some examples, the first stimulationsignal can be a periodic oscillating signal with a first frequency and afirst phase and the second stimulation signal can be a periodicoscillating signal with the first frequency and a second phase,different than the first phase. Additionally or alternatively, in someexamples, the first phase and the second phase can be separated by 180degrees. Additionally or alternatively, in some examples, the firstfrequency can be greater than 150 Hz. Additionally or alternatively, insome examples, the first stimulation signal and the second stimulationsignal can be driven concurrently. Additionally or alternatively, insome examples, the first stimulation signal and the second stimulationsignal can be driven at least partially concurrently with measuring thephysiological signal. Additionally or alternatively, in some examples,the first stimulation signal can be a periodic oscillating signal with afirst frequency and the second stimulation signal can be a periodicoscillating signal with the second frequency, different than the firstfrequency. Additionally or alternatively, in some examples, the firstfrequency and the second frequency can be greater than 150 Hz.Additionally or alternatively, in some examples, the method can furthercomprise: determining, while measuring the physiological signal, that atleast one signal of the one or more signals measured in response to thefirst stimulation signal and the second stimulation signal fails to meetthe one or more criteria; and in response to determining that the atleast one signal of the one or more signals measured in response to thefirst stimulation signal and the second stimulation fails to meet theone or more criteria, ceasing measuring the physiological signal.Additionally or alternatively, in some examples, measuring thephysiological signal can comprise filtering one or more signals measuredby sensing circuitry to remove the one or more signals measured inresponse to the first stimulation signal and the second stimulationsignal from the one or more signals. Additionally or alternatively, insome examples, the method can further comprise: filtering the one ormore signals measured in response to the first stimulation signal andthe second stimulation signal; demodulating the one or more signalsmeasured in response to the first stimulation signal and the secondstimulation signal; windowing the one or more signals measured inresponse to the first stimulation signal and the second stimulationsignal; and/or computing an amplitude of the one or more signalsmeasured in response to the first stimulation signal and the secondstimulation signal. Additionally or alternatively, in some examples, themethod can further comprise: demodulating the one or more signalsmeasured in response to the first stimulation signal and the secondstimulation signal with a first demodulation signal and a seconddemodulation signal, the second demodulation signal 90 degrees out ofphase with the first demodulation signal. A frequency of the firststimulation signal and the second stimulation signal can be the same asa frequency of first demodulation signal and the second demodulationsignal.

Some examples of the disclosure are directed to non-transitory computerreadable storage medium. The non-transitory computer readable storagemedium can store instructions, which when executed by a devicecomprising a first electrode, a second electrode and one or moreprocessing circuits, cause the one or more processing circuits toperform any of the above methods.

Although examples of this disclosure have been fully described withreference to the accompanying drawings, it is to be noted that variouschanges and modifications will become apparent to those skilled in theart. Such changes and modifications are to be understood as beingincluded within the scope of examples of this disclosure as defined bythe appended claims.

The invention claimed is:
 1. A device comprising: sensing circuitryconfigured to sense a physiological signal, the sensing circuitryincluding a first sensing circuit configured to sense a first electrodeand a second sensing circuit configured to sense a second electrode; astimulation circuit configured to drive a stimulation signal on thefirst electrode; and processing circuitry coupled to the sensingcircuitry, the processing circuitry programmed to: detect one or moresignals measured by the first sensing circuit, wherein at least onesignal of the one or more signals is measured by the first sensingcircuit in response to the stimulation signal; detect one or moresignals measured by the second sensing circuit; in accordance with theone or more signals measured by the first sensing circuit and the one ormore signals measured by the second sensing circuit meeting one or morecriteria, measure the physiological signal; and in accordance with theone or more signals measured by the first sensing circuit and the one ormore signals measured by the second sensing circuit failing to meet theone or more criteria, forgo measuring the physiological signal.
 2. Thedevice of claim 1, wherein the one or more criteria includes a firstcriterion that requires the at least one signal measured in response tothe stimulation signal has an amplitude less than a threshold voltage.3. The device of claim 1, wherein the stimulation circuit comprises: asignal generator configured to generate the stimulation signal; and acapacitor configured to couple the stimulation signal to the firstelectrode.
 4. The device of claim 1, wherein the stimulation signal is aperiodic oscillating signal.
 5. The device of claim 1, wherein thestimulation signal has a frequency greater than 40 Hz.
 6. The device ofclaim 1, wherein the stimulation signal has a frequency between 100 Hzand 600 Hz.
 7. The device of claim 1, wherein the sensing circuitryfurther comprises a differential analog-to-digital converter (ADC)configured to convert a differential analog output of a differentialamplifier into a digital output.
 8. The device of claim 1, wherein thesensing circuitry further comprises a saturation detection circuitcoupled to an output of the first sensing circuit and an output of thesecond sensing circuit, wherein the saturation detection circuit isconfigured to detect saturation of the output of the first sensingcircuit or of the output of the second sensing circuit.
 9. The device ofclaim 8, wherein the saturation detection circuit comprises: a firstbuffer coupled to the output of the first sensing circuit; a secondbuffer coupled to the output of the second sensing circuit; amultiplexer coupled to the first buffer and the second buffer, whereinan output of the first buffer and an output of the second buffer arecoupled as inputs to the multiplexer; and an analog-to-digital converter(ADC).
 10. The device of claim 2, wherein the one or more criteriaincludes a second criterion that requires an output of the first sensingcircuit and an output of the second sensing circuit are not saturated.11. The device of claim 1, wherein the sensing circuitry furtherincludes a differential amplifier, wherein an output of the firstsensing circuit is coupled to a first input of the differentialamplifier and wherein an output of the second sensing circuit is coupledto a second input of the differential amplifier.
 12. The device of claim1, wherein the processing circuitry is further programmed to: inaccordance with the at least one signal of the one or more signalsmeasured by the first sensing circuit in response to the stimulationsignal meeting the one or more criteria, cease driving the stimulationsignal.
 13. The device of claim 1, wherein the processing circuitry isfurther programmed to: determine, while measuring the physiologicalsignal, that the at least one signal of the one or more signals measuredby the first sensing circuit in response to the stimulation signal failsto meet the one or more criteria; and in response to determining thatthe at least one signal of the one or more signals measured by the firstsensing circuit in response to the stimulation signal fails to meet theone or more criteria, cease measuring the physiological signal.
 14. Thedevice of claim 1, wherein the stimulation circuit is driving thestimulation signal on the first electrode while measuring thephysiological signal.
 15. The device of claim 14, wherein measuring thephysiological signal comprises: filtering the one or more signalsmeasured by the sensing circuitry to remove the at least one signalmeasured in response to the stimulation signal from the one or moresignals.
 16. A method comprising: receiving a user input requesting aphysiological signal measurement; in response to receiving the userinput, driving a first measurement electrode with a stimulation signal;measuring one or more signals, wherein at least one signal of the one ormore signals is measured in response to the stimulation signal; inaccordance with the one or more signals meeting one or more criteria,the one or more criteria including a criterion that requires the atleast one signal measured in response to the stimulation signal has anamplitude less than a threshold voltage, performing the physiologicalsignal measurement; and in accordance with the one or more signalsfailing to meet the one or more criteria, forgoing the physiologicalsignal measurement.
 17. The method of claim 16, wherein the stimulationsignal is a periodic oscillating signal.
 18. The method of claim 16,wherein the stimulation signal has a frequency greater than 40 Hz. 19.The method of claim 16, further comprising detecting saturation of anoutput of a first sensing circuit coupled to the first measurementelectrode or saturation of an output of a second sensing circuit coupledto a reference electrode.
 20. The method of claim 19, wherein the one ormore criteria includes a criterion that requires the output of the firstsensing circuit and the output of the second sensing circuit are notsaturated.
 21. The method of claim 16, further comprising: in accordancewith the one or more signals meeting one or more criteria, ceasingdriving the first measurement electrode with the stimulation signal. 22.The method of claim 16, further comprising: determining, whileperforming the physiological signal measurement, that the at least onesignal of the one or more signals measured in response to thestimulation signal has an amplitude not less than the threshold voltage;and in response to determining that the at least one signal of the oneor more signals measured in response to the stimulation signal has anamplitude not less than the threshold voltage, ceasing performing thephysiological signal measurement.
 23. The method of claim 16, whereinthe first measurement electrode is driven with the stimulation signalwhile performing the physiological signal measurement.
 24. The method ofclaim 23, wherein performing the physiological signal measurementcomprises: filtering the one or more to remove the at least one signalmeasured in response to the stimulation signal from the one or moresignals.
 25. A non-transitory computer readable storage medium storinginstructions, which when executed by a device comprising a firstmeasurement electrode and one or more processing circuits, cause the oneor more processing circuits to perform a method, the method comprising:receiving a user input requesting a physiological signal measurement; inresponse to receiving the user input, driving a first measurementelectrode with a stimulation signal; measuring one or more signals,wherein at least one signal of the one or more signals is measured inresponse to the stimulation signal; in accordance with the one or moresignals meeting one or more criteria, the one or more criteria includinga criterion that requires the at least one signal measured in responseto the stimulation signal has an amplitude less than a thresholdvoltage, performing the physiological signal measurement; and inaccordance with the one or more signals failing to meet the one or morecriteria, forgoing the physiological signal measurement.